Bug 1615408 Ensure task estimates work when none are selected r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D62816

--HG--
extra : moz-landing-system : lando

tools/tryselect/util/estimates.py

@@ -151,7 +151,8 @@ def find_dependency_durations(task):
         return md
 
     longest_paths = [find_dependency_durations(task) for task in tasklist]
-    return max(longest_paths)
+    # Default in case there are no tasks
+    return max(longest_paths, default=0)
 
 
 def determine_quantile(quantiles_file, duration):